MORE BREAST, PLEASE

Golden, browned skin… tender, succulent flesh… an enticing, irresistible aroma—there’s not much else that people require of their Thanksgiving turkeys! In fact, that beautifully roasted bird on your table is the culmination of a centuries-old undertaking, starting with the first domestication of turkeys by Mesoamerican civilizations in 800 BC.1,2 Since then, generations of selective breeding has solved all the demands of American consumers—of course, excepting the Russian-nesting-doll conundrum that is “turducken”. What’s unexpected though, is that by artificially selecting for the traits that fit our needs, we’ve left the unlucky domestic turkey with some unintended biological consequences…

Building a Better Turkey

While today’s turkey can trace its roots to a domesticated pre-Aztec ancestor, its widespread commercial success is owed to breeds developed in the last century. In 1941, the USDA created Beltsville Small Whites to answer the call for conveniently-sized birds that actually fit in most refrigerators and ovens.3,4 This made turkey a year-round staple in the American kitchen, but people still weren’t satisfied—they wanted more white meat.4 Scientists responded a decade later with the Broad-Breasted White, a breed characterized by enlarged breasts. Since then, this variety has dominated the commercial market, and accounts for nearly all turkeys sold today.4

Broad Breasted Whites take over the commercial market!

Mo' Meat, Mo' Problems

Gravy-lovers and lazy meat-carvers might rejoice at the amount of white meat in modern-day turkeys, but for the birds themselves, abnormally large breasts come at a high cost. These top-heavy turkeys can no longer fly, and often have difficulty walking.5 In addition, with such massive chests, they can’t even get close enough to physically mate with each other.4-7 So in fact, almost all turkeys eaten this Thanksgiving will have come from artificial insemination! And as messy and arduous as that process sounds, it’s not the only indignity that today’s turkeys have to face; despite gains in breast size, these domestic birds have lost volume in a more critical body part—the brain.

The Brain on Domestication

Nearly 3,000 years of careful selection have induced drastic genetic changes within the brains of our delicious, large-breasted friends. When compared to their wild counterparts, the brains of domestic turkeys are visibly smaller, with a startling 30% loss of volume.8 Even more strikingly, this reduction in brain size appears to be a universal trend across a range of domesticated species.8-21 But what do turkeys,8 ducks,9 horses,10 and even rainbow trout11 have in common (both genetically and environmentally), that can explain the prevalence of this phenomenon?

Domesticated llamas also have smaller brains than their wild forebears. Besides an 18% reduction in brain size, these llamas also don't mind being festooned with ear tassels.

One possibility centers on the economic pressures of livestock farming. To increase Feed Conversion Efficiency—the amount (and thus cost) of feed that an animal converts to body weight—substantial genetic changes must occur. For example, turkey farmers can maximize profits with modern breeds that not only grow to market size twice as fast, but also can convert food into body weight more efficiently (20% better than a 1966 breed).22 But if more energy is allotted to the conversion of food to mass, less is available to remaining body functions—including a very energy-demanding brain. In humans at least, the brain comprises only 2% of body weight, but requires 20% of the body’s total oxygen consumption (a measure of basal metabolic rate).23,24 In fact, it’s calculated that in young children, the brain may account for 50% of total oxygen consumption!25 We could then speculate that evolutionary pressures for enhanced weight gain might inadvertently select for smaller (and less energy-demanding) brains.

Another theory for reduced brain size suggests that the process of domestication selects for animals that are easier to handle, specifically those with low reactivity to external stimuli.11 This idea arises, in part, from the increasing evidence that many domestic animals experience disproportionate loss of brain size in areas corresponding to sensory perception.12-15 For example, when measured against their wild ancestors, domestic sheep13 and geese14 exhibit substantial reductions in visual brain structures; likewise, pigs have 32% less auditory cortex than wild boars.15 But others argue that blunted sensory systems could also reflect the ease of life in captivity.16,17 After all, there’s no pressure for evolutionary fitness, if pigs can just laze around in the mud all day instead of honing their senses for finding camouflaged prey and evading sneaky predators. In contrast, homing pigeons require a high level of navigational skills, and were selectively bred for the ability to find their way home. Remarkably, they fly directly home when released at unfamiliar locations (up to 1,100 miles away), and can even do so when anesthetized and thus (presumably) unable to track the outward journey!26 This strong selection criterion led to a domestic breed that—despite having an overall smaller brain—has a larger hippocampus (the brain area involved in spatial cognition and memory) than its wild founders, rock doves.18

Homing pigeons (called carrier pigeons when used for delivering mail) were used extensively during World Wars I and II; they carried critical messages through enemy fire and even took aerial reconnaissance (when strapped with automatically-timed cameras, pictured here). After you read about war pigeons like Cher Ami, you’ll want to hug every pigeon you can find!

Still, how much of these brain changes are genetic versus environmental? Maybe farm life is just boring, and doesn’t provide enough mental stimulation to grow a big brain. This appears to (somewhat) be the case for minks; when raised in broader enclosures, domestic minks had slightly larger brains than their closely caged siblings.20,21 Even so, brains of wild minks still outweighed those of domestics, suggesting that genetics has a stronger effect.20,21 More conclusively, since some domesticated animals escape captivity, study of these feral populations has shown that even after generations of living in the wild, overall brain size is unaffected;27,28 it’s the genetic background that matters most. In the end, it turns out that millennia of human manipulation to the genes of domesticated animals have indeed created long-lasting and dramatic effects—much to our benefit.

Thank You, Science

So for this holiday season, remember to pause and give thanks to the countless animal breeders, artificial inseminators, and research scientists that make a centuries-old tradition, Thanksgiving, possible. And, be grateful that lack of brains doesn’t make your crisp-skinned, juicy-meated turkey any less delicious!